All-ceramic restorations have become a choice for dentists, especially concerning reconstructions based on partially or fully stabilized tetragonal zirconium dioxide. While pure zirconium dioxide has unfavourable mechanical properties, it is possible to control the material by doping it with a stabilising oxide, thus gaining favourable toughness, superior to other dental ceramics. Two currently available zirconia-based ceramics for dental use are partially-stabilized zirconia (PSZ) and yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). Stabilized zirconia can be processed either by soft machining of green-stage or presintered blanks followed by sintering at temperatures varying between 1350° C.-1550° C., or by hard machining of completely sintered blanks. With only one exception known to the authors, all zirconia brands on the market currently are based on Y-TZP. Stabilization of the zirconium dioxide can be made using yttria, magnesium oxide, calcium oxide or cesium oxide. In the case of an addition of yttria this will be added in an amount of 7.5 to 9.5, preferably 8% by weight.
Y-TZP will show different crystal structures depending on temperature. At ambient temperature and up to 1170° C. the crystals have a monocline structure. Between 1170° C. and 2370° C. a phase transformation occurs and it is transformed into a tetragonal structure. Above 2370° C. the material will become converted to a cubic structure. When there is a transformation from tetragonal to monocline structure an increase of volume of the material takes place with about 4.5% which may lead to undesired crack formation within the material. By adding stabilizing oxides, such as CaO, MgO, Y2O3 or CeO2, as mentioned above, the tetragonal phase will become controlled at ambient temperature, a phase which otherwise will not occur at ambient temperature. Y-TZP inhibits actively cracks. When a crack starts to propagate a local tension initiated phase transformation from tetragonal to monocline phase, which leads to an increase of the volume of the crystal structure with 1 to 3% and a directed compressive stress will be obtained which halts the propagating crack.
A number of clinical studies published recent years are based on cemented Y-TZP reconstructions where the cementation techniques rely mainly on macro-mechanical retention. In those cases, the geometry of the supporting teeth gives retention rather than a direct bond between the different structures included (the ceramic material, the cement, dentine and enamel). The geometry needed to enable such retention, however, presupposes tooth preparation with often a substantial tissue loss of tooth material, enamel and dentine, as a consequence. By utilizing bonding technique it would be possible to decrease the need for substantial tooth preparation, thus preserving tooth substance.
The main reason for still being dependent on traditional retentive technologies instead of bonding techniques when using Y-TZP is that the composition and physical properties of the polycrystalline ceramic differ substantially from those of silica based ones. While silica-based ceramics allow for both a micromechanical and chemical bond, the Y-TZP does not include a glass phase. The surface is chemically inert and in most cases show a microstructure that does not allow for micromechanical retention without utilizing some kind of surface modification.
Hydrofluoric etching creates a rough, mainly crystalline surface with pits and micro-lacunas when used to modify the seating surface on dental porcelain and dental glass ceramics. This created surface topography enhances retention by interlocking the luting agent, creating a micromechanical bond. The surface glass is almost completely removed, but the crystal phases are not pronouncedly affected by the acid, and hence remain substantially unchanged after etching. A small portion of glass remnants in the surface, contribute to enhance a chemical bond between the luting agent and the ceram.
The surface of a polycrystalline ceram (e.g. Y-TZP) on the other hand, remains completely unchanged after etching, as the acid does not react with the chemically stable crystals, as previously mentioned.
The interest in a finding a method to obtain strong and reliable bonds between polycrystalline ceramics and a bonding system seem obvious when reviewing the literature. Several methods of surface modifications are highlighted by the current research as for instance silanisation, sandblasting, sandblasting in combination with silanisation, silica coating, tribochemical silica coating, MDP-silane coupling agent surface treatment, selective infiltration etching and different combinations of the methods. Furthermore, several studies have investigated and compared different bonding systems and combinations of primers and luting agents. Novell bonding theories have also been considered, e.g. that chemical bond actually can be achieved to Y-TZP. Still, the literature gives at hand that establishing a strong and reliable bond between Y-TZP and tooth structure, or per definition between Y-TZP and a bonding component is difficult and unpredictable.
Yttrium oxide stabilized tetragonal polycrystalline zirconium oxide (Y-TZP) is an oxide ceramic material having mechanical properties which differs from other oxide ceramic materials. The bending strength of Y-TZP is between 900 and 1200 MPa and the fracture toughness is between 6 and 8 MPa*m0.5, which makes the material suitable as a core material for all ceramic replacements. Outside the core material one or more layers of porcelain is/are added. The porcelain has a considerably lower structural strength, 70 to 120 MPa but is often needed to obtain an esthetically acceptable appearance.
In some papers it is stated that adhesive cementing can be made when the inner surfaces of constructions made of an oxide ceramic material has a micromechanical retention which has been created at the processing. When all ceramic constructions are cut out of a raw material, the cutter leaves a certain structure in the surface, e.g., an inner surface to be applied onto a reduced tooth structure, i.e., onto a dentine structure. This can only be applied at a subtractive production when one cuts the whole replacement from a solid block. At an additive production, one press oxide ceramic powder against a prefabricated surface and then the outer contours are cut and sintered whereby there is no unevennesses by the cutter on the inner surfaces of the construction and thereby limits the possibility of micromechanical retention.